1. Field of Invention
The present invention relates to a complementary metal-oxide-semiconductor (CMOS) sensor. More particularly, the present invention relates to a CMOS sensor fabricated in a substrate having a special shallow trench isolation (STI) structure.
2. Description of Related Art
Charge coupled devices (CCDs) are often employed in digital sensors for image extraction. Its application includes close-circuit TV, camera, and video recorder. However, CCDs are quite costly to produce and bulky. Hence, in order to reduce volume, energy consumption and cost, CMOS photo-diodes that can be formed by semiconductor techniques are a major substitute for CCDs in the future. At present, CMOS photo diodes have been used inside PC cameras and digital cameras.
Photo-diode is a light-sensitive (or light-detecting) semiconductor device that utilizes a P-N junction for converting light energy into electrical signal. Due to the existence of an electric field at the P-N junction, electrons in the N-doped region and holes in the P-doped region cannot diffuse across the P-N junction when no light is shone on the junction. However, when a light beam of sufficient intensity impinges upon the junction region, atoms within the junction region are activated to generate electron-hole pairs. These electron-hole pairs, on reaching the region with an internal electric field, separate from each other. The electrons migrate towards the N-doped region while the holes migrate towards the P-doped region, thereby leading to a current flowing in the P-N junction electrodes. Ideally, the photo diode should be in open-circuit having no electric current flowing when the device is in the dark.
Conventionally, a field oxide (FOX) layer is used as a device isolating structure for a CMOS sensor. FIG. I is a cross-sectional view of a conventional CMOS sensor structure of P-N photo-diodes.
The CMOS sensor is fabricated in several steps. First, a substrate 100 such as a P-type silicon substrate is provided. Then, a field oxide layer 102 is formed over the substrate 100 using a local oxidation method, wherein the field oxide layer 102 serves as a device isolation structure. Thereafter, a P-well is formed in the substrate 100, and then a gate structure 106 is formed over the substrate 100. The gate structure 106 includes a gate electrode 114 and a gate oxide layer 112. In the next step, the gate structure 106 is used as a mask for implanting N-type ion into the substrate 100 to form a lightly doped region. Subsequently, spacers 122 are formed on the sidewalls of the gate structure 106. Next, the gate structure, the spacers 122 and a patterned photoresist layer are used as a mask for implanting N-type ions again into the substrate 100 to form a heavily doped region, and then the photoresist is removed. The lightly doped region and the heavily doped region together form source/drain regions 108 and 118. After that, another photoresist layer is formed over the substrate 100 exposing the desired sensor region, and then another light N-type ion implantation is carried out to form a sensor region 128 encompassing the source/drain region 118.
Since a P-N junction is formed around the border of the sensor region 128, electron-hole pairs are generated when a light beam is incident on this region. The light energy is then transformed into an electrical signal. On the contrary, if the incoming light incident on the substrate 100 without passing through the sensor region 128, no electrical signal is to be generated. It is often that the light passing through the sensor region 128 is commonly dim to only produce a small electrical signal. Consequently, the contrast ratio of this photo-diode device is rather low, in other words, this type of device is not sensitive enough.
Moreover, as chip size continues to shrink, isolating devices with a field oxide layer is infeasible. This is because a field oxide layer occupies a relatively large surface area plus other problems such as bird's beak encroachment.